This invention relates to an epoxy resin composition of self-polymerization or phenolic curing type, a laminate film using the same and suited for semiconductor packaging, and a semiconductor device sealed with the laminate film.
The recent trend of electronic equipment is characterized by high speeds of size and weight reduction. Under the circumstances, it is desired to reduce the size and weight of semiconductor devices serving as the brain of electronic equipment. The ultimate target is to reduce the packaging area to the chip size, realizing a chip size package (CSP).
Epoxy resins are widely used in constructing semiconductor packages because of their good adhesion, heat resistance and moisture resistance. As the package system is diversified as mentioned above, epoxy resins are now used not only as the encapsulants well known in the art, but also as coating, die bonding and underfill materials. Most such materials are diluted with solvents or in paste form.
Of these, the solvent-diluted materials require an attention to carefully remove the solvent that volatilizes off on use, from the standpoints of the health maintenance and safety of workers, undesirably adding to the cost.
Coating materials are used in several ways. One proposed method is to cover the surface of a semiconductor chip solely with a thin film of an epoxy resin composition to effect sealing. It is also proposed to screen print an epoxy resin composition in paste form. These materials include paste-like epoxy resin compositions such as those of the acid anhydride curing type and the amine curing type which are well known in the art. Alternatively, these paste-like epoxy resin compositions may take the form of a film which has been converted to B-stage.
Most of the prior art die bonding and underfill materials were paste-like epoxy resin compositions. Recently, films formed from paste-like die bonding materials are widely utilized. On use, a film is pressed against a lead frame or substrate, and a semiconductor chip is joined onto the film whereupon the film is cured. Most of these die bonding materials are epoxy resin compositions of the acid anhydride curing type and the amine curing type which are well known in the art.
These B-staged films are easier to handle than paste-like materials and provide a very promising system that contributes to the simplification of a semiconductor device fabricating process. However, the epoxy resin compositions of the acid anhydride curing type and the amine curing type are insufficient in the storage of uncured compositions and the moisture resistance and high-temperature performance of cured compositions. They are unsatisfactory in directly covering the semiconductor chip surface with a thin film.
As compared with the epoxy resin compositions of the acid anhydride curing type and the amine curing type, epoxy resin compositions of the self-polymerization type, especially those using imidazoles as the curing catalyst, and epoxy resin compositions of the phenolic curing type are satisfactory in the storage of uncured compositions and the moisture resistance and high-temperature performance of cured compositions, but are difficult to control the progress of reaction and hence, to form B-staged films. To obtain flexible, easy-to-work films, the softening point of an epoxy resin base and a phenolic resin curing agent must be lowered. Undesirably, this is done at the expense of the heat resistance of cured compositions.
To solve the above problem, we proposed in U.S. Ser. No. 09/404,301 U.S. Pat. No. 6,210,811 an epoxy resin composition comprising (A) an epoxy resin containing up to 10% by weight of a 2-nucleus compound and at least 50% by weight of 3 to 5-nucleus compounds combined and having a dispersity of up to 1.7, (B) an inorganic filler, and (C) a curing catalyst, the epoxy resin composition in an uncured state having a glass transition temperature of lower than 15xc2x0 C.
Even an epoxy resin composition of self-polymerization or phenolic curing type can find a good compromise between the flexibility of uncured composition and the heat resistance of cured composition as long as the dispersity or molecular weight distribution of the epoxy resin is properly controlled.
Regrettably, a film material in the form of an epoxy resin composition of self-polymerization or phenolic curing type becomes extremely low in melt viscosity upon heating because both the epoxy resin and phenolic resin used as constituent components are of low molecular weight. Then, when the film is moderately attached to a substrate or board under pressure or when the film is then cured, there will occur, depending on the shape of a device or package and the moderate pressing and curing conditions of the film, a problem that the film loses its original shape or entrapped voids are formed near irregularities on the device.
An object of the invention is to provide an epoxy resin composition of the self-polymerization type or the phenolic curing type which, owing to the addition of a thermoplastic resin of linear molecular structure having a specific molecular weight, avoids the problem that when the gap between a semiconductor chip and a substrate or the surface of a semiconductor chip is sealed with an uncured film of the composition, the film. will lose its original shape or entrapped voids are formed near the chip, and which has a low glass transition temperature in an uncured state while maintaining the heat resistance, moisture resistance and low stress of a cured composition.
Another object of the invention is to provide a laminate film comprising a flexible, easy-to-work thin layer of the epoxy resin composition.
A further object of the invention is to provide a semiconductor device in which the gap between a semiconductor chip and a substrate or the surface of a semiconductor chip is sealed with the laminate film.
The invention is directed at an epoxy resin composition comprising an epoxy resin containing up to 10% by weight of a 2-nucleus compound and at least 50% by weight of 3 to 5-nucleus compounds combined and having a weight average molecular weight (Mw)/number average molecular weight (Mn) ratio (referred to as a dispersity or Mw/Mn, hereinafter) of up to 1.7, an inorganic filler, a curing catalyst, and optionally a phenolic resin, and having a glass transition temperature (often abbreviated as Tg) of lower than 15xc2x0 C. in an uncured state. When the composition is of the self-polymerization type, an imidazole is preferably used as the curing catalyst. When the composition is of the phenolic curing type, a phenolic resin having a Tg of lower than 15xc2x0 C. and free of a monomer is preferably used as the curing catalyst. Further preferably, a copolymer obtained by reacting an aromatic polymer with an organopolysiloxane is blended in the composition. Still further preferably, the composition has a volatile content of up to 0.1% by weight. We have found that by compounding a thermoplastic resin, especially a thermoplastic resin of linear molecular structure having a number average molecular weight of 5,000 to 200,000 in any of the foregoing compositions, the resulting composition is effective in forming a film which is very easy to work at room temperature by virtue of its good elasticity and flexibility. By enclosing a semiconductor chip with the film, there is obtained a semiconductor device having improved heat resistance, improved moisture resistance, low stress property, and a minimized void content. The compounding of the thermoplastic resin can avoid the resin flow and short molding problems. The invention is predicated on these findings.
In a first aspect, the invention provides an epoxy resin composition comprising (A) an epoxy resin containing up to 10% by weight of a 2-nucleus compound and at least 50% by weight of 3 to 5-nucleus compounds combined and having a dispersity of up to 1.7, (B) an inorganic filler, (C) a curing catalyst, (D) a thermoplastic resin, preferably a thermoplastic resin of linear molecular structure having a number average molecular weight of 5,000 to 200,000, and optionally, (E) a phenolic resin. The epoxy resin composition in an uncured state has a glass transition temperature Tg of lower than 15xc2x0 C.
When the phenolic resin (E) is omitted, the epoxy resin composition is of the self-polymerization type wherein the curing catalyst (C) is preferably an imidazole. When the phenolic resin (E) is included, the epoxy resin composition is of the phenolic curing type. The phenolic resin preferably has a glass transition temperature of lower than 15xc2x0 C. and is free of a monomer. The epoxy resin composition may further contain (F) a copolymer obtained by reacting an aromatic polymer with an organopolysiloxane of the following compositional formula (1):
RaRxe2x80x2bSiO(4-a-b)/2xe2x80x83xe2x80x83(1)
wherein R represents hydrogen, an amino, epoxy, hydroxyl or carboxyl group-bearing monovalent hydrocarbon group, or an alkoxy group, Rxe2x80x2 represents a substituted or unsubstituted monovalent hydrocarbon group, xe2x80x9caxe2x80x9d is a positive number of 0.001 to 1, xe2x80x9cbxe2x80x9d is a positive number of 1 to 2, and the sum of xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d is from 1 to 3, the number of silicon atoms in one molecule is 2 to 1,000, and the number of functional groups R directly attached to silicon atoms in one molecule is at least 1. Further preferably, the epoxy resin composition in an uncured state has a volatile content of up to 0.1% by weight.
In a second aspect, the invention provides a laminate film comprising at least two layers, at least one layer being formed of the epoxy resin composition to a thickness of 20 to 150 xcexcm.
In a third aspect, the invention provides a semiconductor device comprising a semiconductor chip having a surface sealed with the laminate film in a cured state.